[fix] net_layer_blob tutorial file

I found some typos in `docs/tutorial/net_layer_blob.md` file, and I also added some additional information myself. I would like you to check whether my change from 1x1x1000x4096 to 1x1x1000x1024 in `Blob storage and communication` (next to last paragraph) is correct, or there is something that I'm missing.

I would also like to read/check the other tutorial files in the next few days, so please do not merge right now.